316 CHELONIA 



CHAP. 



ends of the iliac bones ; but since these find a much more effective 

 support in the shell, the distal ends of the sacral vertebrae 

 fuse with the eighth, or so-called last, pair of costal plates. 



The neural arch of the ninth vertebra rests upon its 

 centrum; but the neural arches of the other trunk -vertebrae, 

 although long, rest upon two centra ; retaining, like the ribs, 

 their original intercentral position ; and in most cases the 

 neuro-central sutures remain throughout life. The atlas and 

 the last cervical vertebra deserve special attention. In many 

 tortoises, e.g. Trionyx, Clemmys, Testudo, the three constituent 

 parts of the atlas, namely, the neural arch, the centrum, and 

 the intercentrum or first pair of united basiventralia, do not 

 ankylose, but remain loosely connected ; and the first centrum, 

 instead of forming an odontoid process, remains movably attached 

 to the second centrum, although it sometimes carries, and fuses 

 with, the second intercentral piece. In other tortoises, e.g. 

 Platemys and Chelys, however, all the parts of the atlas co-ossify 

 and form a complete, solid vertebra which 

 articulates by a concavo-convex joint with 

 the centrum of the second vertebra. The 

 normal number of cervical vertebrae is 

 eight in all Chelonians. The first spinal 

 nerve issues between occiput and atlas, 

 all the others behind the neural arches of 

 their vertebrae. The last, or eighth cer- 

 hurum. The second basi- vical, owing to the retractility of the neck, 



ventral (white) is attached P i ~\ '*.!_ i -J.-L. 



to the posterior end of the forms elaborate joints ; its centre fits with 

 first centrum, which, not a knob into a cup of the ninth, and its 



being fused with the second , / i i i 



centrum, is not yet an post-zygapophyses form broad, curved 

 odontoid process. 2, The articulating concave facets for the recep- 



complete atlas of an adult , . . . 



Trionyx gangeticus, still tion of the anterior zygapophyses of the 

 ^tt* 11 / t f mil , os P nfl y lous - fixed ninth vertebra. In the Trionychidae 



3, The first and second cer- * 



vical vertebrae of an adult the zygapophyses are most elaborate, and 

 Platemys 4, The > complete th alone art i cu i ate with the ninth ver- 



atlas of a Chdys fimbriata. J 



tebra, while the centra do not join, but 



remain, or rather become, separated by partial resorption. In 

 the Chelonidae, in conformity with the non-retractile and short 

 neck, all the cervical joints are much reduced. 



The skull (cf. Fig. 54, H, I, K, p. 280) agrees fundamentally 

 with that of Sphenodon and of the Crocodilia, but it is 



